The present invention relates generally to DC-DC and DC-AC power conversion in AC drive systems and, more particularly, to a system and method for reducing DC link current ripple in such drive systems.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
In energy systems of hybrid and electric vehicles, energy is transferred from the energy storage device to a DC link, with the DC link being electrically coupled to a DC-AC inverter and the electric motor. Often, a bi-directional boost converter is coupled to the DC link to boost voltage from the energy storage device on a low voltage side of the energy system to a higher level voltage on a high voltage side of the energy system for supply to the DC-AC inverter. A typical system involving a battery, DC-DC converter, DC-AC inverter and motor is shown in FIG. 1. As seen therein, a DC link capacitor is required to absorb current ripple created during the inversion from DC to AC. The DC link capacitor is a critical and bulky component of the system, and the DC link capacitor is typically sized for the maximum ripple current it sees—with the life of the capacitor depending on the current through it over time. In automotive applications where the ambient temperature is high, reduction in capacitor ripple current is crucial.
However, while it is recognized that reducing the capacitor size and increasing its life is crucial in low cost, compact power conversion applications, not much work has been done in the area of capacitor minimization with DC-DC-AC power conversion schemes. In work done to date for such capacitor minimization, prior art advancements have obtained ripple reduction for only a very narrow band of duty ratio, used a bang-bang controller to reduce ripple current and reduce capacitor size at the expense of having a larger DC voltage ripple, and/or by using two inverters to cancel the current ripple, such that a motor with segmented windings or two separate motors are required.
It would therefore be desirable to provide a system and method for reducing DC link current ripple in a drive system that includes a DC-DC converter and DC-AC inverter. Reduction of the DC link current ripple would allow for the reduction of the DC link capacitor size and increase the life of the capacitor.